Is information shared with a children's aid society by one parent confidential from the other parent?

Not necessarily, and not in the way many parents assume. Information a parent shares with a children's aid society isn't automatically privileged the way, say, a conversation with your own lawyer is - the society is investigating a child's safety, not acting as either parent's confidential advisor, and it can share relevant information with the other parent, other professionals, or a court where doing so serves the child's protection or a related legal proceeding. A parent shouldn't assume that whatever they tell a worker stays fully confidential from the other side.

That said, societies do have their own confidentiality practices and generally won't hand over an entire file informally - the other parent typically needs a proper request or court order to see specific records, as with any CAS disclosure. So there's a middle ground: the information isn't secret from the system as a whole, but it also isn't simply available to the other parent on demand. If you're speaking with a society and are concerned about how something you say might be used in your parenting case, get legal advice before the conversation where possible, and be accurate and consistent regardless, since inconsistency between what you tell the society and what you say in family court can itself become an issue.